1. Field of the Invention
The present invention relates to a novel Group IV elements-bridged metallocene complex, and more particularly, to a Group IV element-bridged metallocene complex which has two catalytic sites and is capable of preparing olefin polymers having a bimodal molecular weight distribution. The Group IV elements include Group IVA elements and Group IVB metals.
2. Description of the Prior Art
Polyolefins, which include high density polyethylene (HDPE), are some of the most versatile thermoplastic resins and they have been used in a wide range of applications.
Most of the physical and mechanical properties of polyolefins, such as the high strength and high impact, stress, and puncture resistances, together with high toughness, are attributed, at least in part, to their relatively high molecular weight. However, as the molecular weight of the resin increases, the processability of the resin usually decreases.
In recent years, it has been found that polyolefins having a multimodal (typically bimodal) molecular weight distribution (MWD) can, not only retain the advantageous properties associated with high molecular weight, but also exhibit improved processability.
A bimodal MWD polymer (which can be also simply referred to as "bimodal polymer") is defined as a polymer having two distinct molecular weight distribution curves as observed from the gel permeation chromatography (GPC). In other words, a bimodal polymer can be considered as a mixture containing a first polymer with a relatively higher molecular weight and a second polymer with a relatively lower molecular weight that are blended together.
Various approaches have been disclosed for producing bimodal polyolefins. The simplest approach is to physically blend together two polymers having different molecular weights. However, this simplistic approach suffers from the problem that only with polymers that can be completely molten, homogenization can be obtained. If one of the polymers is not completely molten, then the polymer blend will be inhomogeneous. This can cause a myriad of problems.
U.S. Pat. Nos. 5,284,613 and 5,338,589 disclose a two stage polymerization process for preparing a bimodal polyolefin. In the first stage, olefin monomers are contacted with a catalyst under polymerization conditions to produce a relatively high molecular weight (HMW) polymer powder, wherein the polymer is deposited on the catalyst particles. In the second stage, the HMW polymer powder containing the catalyst is further polymerized with additional olefin monomers to produce a relatively low molecular weight (LMW) polymer much of which is deposited on and within the HMW polymer/catalyst particles from the first. The disadvantages of such a two stage process are that two reactors are needed, resulting in undesirably high capital investment.
U.S. Pat. No. 5,369,194 discloses a process for preparing bimodal polyolefins in a single reactor. The catalyst system so used includes two different transition metal catalysts supported on the same solid support material. Therefore, high and low molecular weight polymers can be formed on the same catalyst particle. The shortcoming is that procedures for preparing the solid support material which is supported with two different catalysts is complicated and difficult. Moreover, the preferable activities for the two different catalysts may be at different conditions. Therefore, when one catalyst is activated, the other catalyst may be inactivated.